Low-pressure fluorescent tubes, so technical solutions for their electronical starting and powering are well known and have been widely used for several decades. Most of prior art solutions are designed and build as an ON/OFF (two-state) switch for the sake of simplicity and low-cost: the fluorescent lamp is either switched on or switched off and their light input intensity cannot be dimmed neither within ranges nor continuous.
Low-pressure fluorescent lamps and ballasts for their electrical starting and powering are well known technology and various types of fluorescent lamp and ballast combinations have been widely used for several decades. Most older magnetic ballasts contain passive components only, provide voltage outputs in the 50 Hz to 60 Hz frequency range are subject to start-up and running flicker and operate extremely inefficiently. Newer electronic ballasts utilize active electronic switching circuits, provide high-frequency voltage outputs in the 20 kHz to 50 kHz frequency range and operate more efficiently. However, most electronic ballasts are very simple in design, have insufficient input protection and radio-frequency filtering, operate at poor input power factors, inject large amounts of low-frequency harmonics back into the ac supply, have square-wave voltage outputs, utilize feed-back control to maintain the output voltage constant and are suitable only to operate in the ON/OFF mode (i.e.: non-dimming). Such ballasts eliminate start-up and running flicker but do not operate very reliably and, in addition, may cause severe reductions in lamp lifetimes since it is well known that one of the major causes of reduced lamp lifetime is the result of subjecting the lamp filaments to high-voltage square-wave excitation. Some of the latest types of electronic ballast designs have attempted keep the excitation quasi-sinusoidal by resonating the output square-waves with series resonant circuits. The load impedance of the lamp in such circuits is generally an integral part of the series resonant circuit and different ballasts are required to power fluorescent loads with different impedances. Output voltage is sensed and feedback is used to modulate the square-wave to maintain output voltage relatively constant during load impedance changes. If, during normal ON operation, a lamp load is removed from such a circuit, the output voltage across the remaining lamp and across the feedback circuit may become very high and result in the damage of the remaining lamp, the converter's electronic circuits and also pose a potential risk to the user.
As an example of latest technology, U.S. Pat. No. 4,933,605, discloses a high frequency dimmable electronic ballast circuit for low-pressure fluorescent lamps. The circuit has a dc power supply, a converter connected to the power supply for preparing a square-wave ac voltage, a series-resonant output circuit with the fluorescent lamp load connected directly in series (or via a transformer) with the resonant circuit. Accordingly, the lamp is an integral part of the resonance circuit used to force the waveform of the voltage between the lamp filaments to be quasi-sine wave in shape. The ballast uses feedback control to try to keep output voltage essentially constant and it is claimed that the ballast is capable of driving more than one fluorescent lamp. Although this ballast appears to provide significant advantages over most prior types, it is designed to operate correctly only with designated types of lamps and various different ballast must be used to supply different lamp types. In addition, since the lamps are active parts of the resonance circuit, if lamp impedances increase the peak values of the voltage between the filaments will also tend to increase. Impedance increases due to aging may tend to accelerate the lamp aging process and the failure of one lamp may cause the other to also fail. In addition, removal of a lamp during normal ON operation may result in the generation of very high voltages that could cause damage to the remaining lamp and/or the converter circuit and may also be dangerous to the user.
It is the object of the present invention to provide a high-frequency dimmable electronic ballast that will operate consistently regardless of source and load changes and is suitable to power various types of fluorescent lamps. Further, during the design process, it was recognized that by ensuring a constant amplitude sinusoidal output regardless of loading and at the same time fully isolating the source from the load, high-voltage caused failures of the control circuits would be virtually eliminated. Accordingly, it is also the object of the present invention to provide a high-frequency dimmable electronic ballast that does not require feed-back from the output in order to maintain the output voltage constant. Finally, it is also the object of the present invention to provide a high-frequency dimmable electronic ballast capable of powering low-pressure fluorescent lamp loads while meeting all "state-of-the art" input and output requirements by providing input and output short circuit and overtemperature protection, reducing conducted and radiated radio-frequency interference (RFI), improving efficiency and input power factor, reducing input total harmonic distortion (THD) and limiting no-load output peak voltages. By meeting the above objectives this ballast will be easier and safer to use and will operate more reliably than other types presently available.